1. Field of the Invention
This invention relates to a solvent adsorber and a batch type solvent recovery system for recovering solvent from solvent-containing gases by the use of such a solvent adsorber, and more particularly to solvent recovering techniques which obviate the use of steam in the regenerating stage. The present invention can be suitably applied to removal and recovery of FREON type solvents such as chlorofluorocarbon (CFC), hydrogen chlorofluorocarbon (HCFC), and fluorocarbon (FC); chloric solvents such as methylene chloride, 1-1-1 trichloroethylene, perchloroethylene, and trichloroethylene; and organic solvents such as toluene, xylene, benzene and alcohol.
2. Description of the Prior Art
Recently, environmental pollution has become a matter of great concern, with a trend of prohibiting the release of carbonaceous waste into the atmosphere by tightening related regulations from the standpoint of conservation of the environment. Above all, atmospheric pollution with FREON gases containing chlorides or with chloric solvents has become a serious problem on a global scale, urging restrictions on the production of FREON gases or the like. With regard to the restriction of the FREON gas production, although the total prohibition of use might be called for in the future, what is considered to be the best measure at the present stage or until development of a suitable replacement is to resort to a closed system which is adapted to recover and reuse a FREON gas which has thus far been released into the atmosphere in case of prior art systems. For this purpose, there have been proposed and used various types of solvent recovery systems.
Shown in FIG. 10 in a block diagram is a conventional batch type solvent recovery system, which can be employed for recovering various industrial solvents in addition to FREONs or other chloric solvents under different conditions. Here, for the convenience of explanation, the system is regarded as a recovery system for FREON, a representative one among various chlorinated hydrocarbon gases. Namely, the FREON recovery system is provided with first and second adsorption towers 121 and 122 in which batchwise adsorption and desorption or release of FREON gas are alternately repeated to recover FREON gas continuously therefrom. These adsorption towers 121 and 122 are packed with FREON adsorbent like activated carbon.
FREON containing air is introduced into the adsorption towers 121 and 122 by a blower 127a through a pipe 141 and its branch pipes (141a and 141b) which are provided with on-off valves 128 and 129, respectively. These on-off valves 128 and 129 control the flow of FREON-containing air to be supplied to the adsorption towers 121 and 122, respectively. Cooling air is also supplied to the adsorption towers 121 and 122 by a blower 127b through a pipe 142 and its branch pipes (142a and 142b). These pipes 142a and 142b are provided with on-off valves 130 and 131 which control the flow of cooling air to the adsorption towers 121 and 122, respectively.
Steam 125, is supplied to the adsorption towers 121 and 122 through branch pipes 143a and 143b of piping 143, which are provided with on-off valves 134 and 135 to control the supply of regenerating steam to the adsorbent in the adsorption towers 121 and 122, respectively.
On the other hand, pipes 144a and 144b which are connected to the gas outlets of the adsorption towers 121 and 122 are jointly connected to a pipe 144. On-off valves 132 and 133 are inserted in the pipes 144a and 144b to control the flow of cleaned air from the adsorption towers 121 and 122, respectively. Cleaned air is released to the atmosphere through the pipe 144.
Pipes 145a and 145b which are connected to the other gas outlets of the adsorption towers 121 and 122 are jointly connected to a pipe 145. On-off valves 126 and 137 are inserted in the pipes145a and 145b to control the discharge of post-regeneration gas from the adsorption towers 121 and 122, respectively. The pipe 145 is connected to a condenser 138 which is supplied with cooling water to condense the post-regeneration gas. The resulting liquid is fed to water separator 139 through a pipe 146 to separate water from liquid FREON. This liquefied FREON is sent to and stored in a storage tank 140 through a pipe 147.
The above-described batch type FREON gas recovery system operates in the manner as follows.
Firstly, let us assume that the recovery system is now in a phase of operation where the adbsorbent packed in the first adsorption tower 121 has been regenerated into activated state while the adsorbent in the second adsorption tower 122 has been fully laden with adsorbed FREON in that case, the on-off valves 128 and 132 are open, the on-off valves 129 and 133 are closed, the on-off valves 130 and 131 are closed, the on-off valves 134 and 136 are closed, and the on-off valves 135 and 137 are open. Under these conditions, FREON containing air 123 is fed to the adsorption tower 121 by the blower 127a through the pipe 141a, and stripped of FREON by adsorption as it is passed through the adsorbent in the tower 121. Resulting cleaned air 126 is discharged from the adsorption tower 121 through the pipe 144a.
On the other hand, steam 125 is supplied to the second adsorption tower 122 through pipe 143b and passed through the adsorbent in the second tower 122, whereupon FREON and water which were adsorbed on the adsorbent in a previous adsorption stage are heated and released from the adsorbent by the energy of steam. Released FREON and water are sent to the condenser 138 together with steam and cooled there. As a result, the FREON gas and water are liquified to form a mixture liquid of FREON and water, which is then sent to the water separator 139 to separate water from liquid FREON. The resulting liquid FREON is collected in the storage tank 140.
Nextly, after a lapse of a predetermined time, the on-off valves 135 and 137 are closed to end the desorption stage of the second desorption tower 122 while continuedly hoding the first adsorption tower in the adsorption stage with the on-off valves 128 and 132 in open state. Then, the on-off valves 131 and 133 are opened to supply the adsorption tower 122 with cooling air which is almost or completely free of FREON. The cooling air is passed through the adsorbent in the adsorption tower 122 to dry and cool off the adsorbent of activated carbon or the like which was moistened with steam in a previous desorption stage. The spent cooling gas fromthe adsorption tower 122 is released into the atmosphere together with cleaned air through pipes 144b and 144.
Upon a lapse of a predetermined time, the on/off valves 129, 134 and 136 are opened, and the on-off valves 128 and 132 are closed, holding the on-off valve 133 in open state and the on-off valves 130, 135 and 137 in closed state.
As a result, the FREON-containing air 123 is passed through the adsorbent in the second adsorption tower 122 for the FREON adsorption, while steam is passed through the adsorbent in the first adsorption tower 121 for FREON desorption. The exhaust gas which contains a FREON and moisture is sent to the condenser 138 and water separator 139 to recover the FREON therefrom.
After a lapse of predetermined time, the on-off valves 134 and 136 are closed and the on/off valves 130 and 132 are opened to supply cooling air 124 to the first adsorption tower 121. Accordingly, after the desorption stage, the first adsorption tower 121 proceeds to a cooling (drying) stage.
In this manner, a liquid FREON is recovered from FREON-containing air 123 by a cycle of operation consisting of adsorption, desorption and cooling stages which are established by switching the respective on-off valves. While the first adsorption tower 121 is in the adsorption stage, the second adsorption tower 122 is put in operation of the desorption and cooling stages; and while the second adsorption tower 122 is in the adsorption stage, the first adsorption tower 121 is put in operation of the desorption and cooling stages. Consequently, FREON gas can be continuously recovered from exhaust air. Where the continuous recovery of FREON is not a mandatory requisite, the adsorption, desorption and cooling stages may be conducted at certain time intervals by the use of a single adsorption tower. In a case where the operating time of the adsorption stage does not conform with the combined operating time of the desorption and cooling stages, there may be provided three or more adsorption towers.
Heretofore, activated carbon in the form of pellets or of granular or fibrous form has been used in the batch type FREON gas recovery system as described above (Japanese Laid-Open Patent Application 61-174923). More specifically, it has been the general practice to adsorb FREON on activated carbon py passing FREON containing gas through activated carbon particles having a grain size of 0.5-10 mm or through felt of fine activated carbon fiber, without using an adsorbent material in the shape of a molded monolithic structure.
In this regard, there has been a problem that generation of acids is unavoidable in the stream desorption process where chloride-containing organic solvents like FREON are desorbed with steam. Namely, although the FREON is an extremely stable substance which is mainly composed of carbon, chlorine and fluorine, the adsorbed FREON on activated carbon partly decomposes if exposed to steam for a long time period. Therefore, repeated recovery and reuse of FREON under such conditions will invite degradations in purity of the recovered FREON and increases in acid concentration to such a degree as to make the recovery system practically incapable to serve for the intended purpose. Besides, the entrainment of acids in exhaust water or in cleaned gas might cause secondary pollution.
Further, when recovering a chloric solvent which contains a large quantity of carcinogenic substance as a stabilizer, it becomes necessary to provide additional equipment for waste water treatment to prevent environmental pollution by a water-soluble stabilizer component which dissolves into water and might otherwise be mostly discharged together with waste water or by a chloric solvent component which might dissolve into waste water as a result of contact of the solvent with steam.
Furthermore, in a case where steam is employed as a regenerating gas, there has to be additionally provided piping for steam and a steam boiler. Thus, such solvent recovery system increase the necessary equipment, maintenance and running costs. Besides, the steam desorption process in which basically water remains on the surface of the adsorbent needs to provide a drying stage to remove moisture from the adsorbent surface prior to an ensuing adsorption stage. However, due to insufficient drying of the adsorbent, the prior art recovery system fails to use its originally designed adsorption capacity to its full extent and therefore requires an objectionably large amount of adsorbent.
For the reasons stated above, the prior art solvent recovery system which uses steam has a number of drawbacks which are related with water.